An air-conditioning system in which an outdoor unit and an indoor unit are connected to each other in a manner capable of transmitting information via a transmission line has been known conventionally. The system is configured such that various types of signals such as control signals are transmitted and received via the transmission line. In such an air-conditioning system, as various types of signals are attenuated according to distance, a transmission relaying apparatus is interposed in the transmission line to perform a relaying process such as shaping of signals being transmitted on the transmission line. The transmission relaying apparatus includes a comparator (H/W) configured to compare a signal flowing on the network with a predetermined reception threshold voltage, and determine whether to perform shaping and relaying of a received waveform.
Here, a controller of a refrigeration cycle apparatus configured to determine a margin with respect to a production standard such as confirmation of a transmission line length, in the air-conditioning system described above, has been proposed (see Patent Literature 1, for example). Patent literature 1 discloses a controller of a refrigeration cycle apparatus configured to transmit a small signal slightly larger than a standard value and compare a received signal, when the small signal is received, with a preset receiving threshold to thereby determine whether the state of the transmission line is normal and check whether there is a margin with respect to the environment such as noise.